[PHI 3000] One's modus ponens is another's modus tollens

The skeptic argues as follows:

1. If I don't know that I am not a brain in a vat, then I don't know that I have hands.
2. I don't know that I am not a brain in a vat.
3. Therefore, I don't know that I have hands.

G. E. Moore responds to this skeptical argument as follows:

1. If I don't know that I am not a brain in a vat, then I don't know that I have hands.
2. I know that I have hands.
3. Therefore, I know that I am not a brain in a vat.

Both arguments are valid. Which do you find more convincing?